President Trump and North Korean leader Kim Jong Un recently dampened their threats to unleash nuclear “fire and fury,” but they remain on a collision course. With his bluster that North Korea will never be allowed to field a missile capable of striking the U.S. homeland with a nuclear warhead, Trump may have red-lined himself into a box that will require Houdini-like contortions to escape.

If Kim calls his bluff, Trump has three choices: He can back down, with a tremendous loss of regional credibility, inviting further Kim escapades and encouraging South Korea and Japan to consider acquiring their own nuclear arsenals. He can assure the American people that our missile defense interceptors can reliably shoot a missile down, which would be a lie. Or he can launch a preventive conventional or nuclear strike against North Korea’s nuclear missile launch sites. This act of war would probably trigger a full-scale conflict resulting, especially in the nuclear scenario, in heavy casualties and massive destruction throughout the Korean Peninsula and in Japan, some of it caused by the North’s surviving nuclear-tipped missiles fired in retaliation.

I am a former nuclear missile launch officer. And to me, a preventive nuclear strike would represent a breach of contract with our nuclear armed forces, who would carry it out. That concern is worse under Trump and his brinkmanship, but it goes beyond him: The protocol for ordering the use of nuclear weapons endows every president with civilization-ending power. The crisis with North Korea shows just how urgent it is that we change the way the United States handles its nuclear arsenal.

Every American nuclear commander makes a pact with the devil by agreeing to obey orders to fire weapons that may kill millions of people in retaliation for a nuclear attack upon the United States or our allies. During my training in 1971 to become a nuclear missileer responsible for launching up to 50 Minuteman intercontinental missiles, it was instilled in me and my crewmates that the central purpose of nuclear weapons is deterrence. Our job was to prevent our adversaries from nuking us first by convincing them that we would not hesitate to turn our firing keys and obliterate them in a second strike.

There is, of course, far more to the story, and we knew it. Deterrence could fail by design or accident. Who could be certain that a reckless leader would not intentionally launch a nuclear strike? (We kept this concern to ourselves to avoid censure or court-martial — or discharge, the fate of one brave questioner, Maj. Harold Hering .) Or that nuclear war might not erupt as a result of unauthorized actions, accidental detonations or a false warning of an incoming attack? We were on board for hitting back in retribution not only at the enemy’s economy (mostly located in cities) and its leadership, but also at its nuclear forces to diminish its ability to cause further destruction. This mission went beyond deterrence and into the realm of nuclear warfighting.

The vast bulk of our targets were (and still are) opposing nuclear forces, and we would have willingly tried to destroy them along with Soviet, Chinese or North Korean leadership in wartime if deterrence failed and the enemy struck first. We knew we might try to launch first if an enemy nuclear strike appeared to be imminent — minutes or hours away, not months or years. Some of our nuclear war plans had “preemptive” strike designators, which meant we would try to beat the adversary to the punch when intelligence and surveillance clearly indicated an imminent and irrevocable strike against the United States.

We even accepted that we might be pawns in a game of crisis brinkmanship, in which U.S. nuclear forces would be put on higher alert, mobilized and dispersed to make an adversary fear an imminent attack and back down from taking or even entertaining further aggressive action. I once participated in such “risk manipulation” during the 1973 Arab-Israeli war, when the White House ordered us to prepare to fire our rockets at the Soviet Union, whose leader threatened to send troops to help the Arab nations. As U.S. strategic bombers taxied to runways and submarines moved to launch depth, I and other missileers retrieved our launch keys and codes from the safes in our underground bunkers and strapped into our chairs to brace for an imminent nuclear exchange. This turned out to be Kissingerian theater (President Richard Nixon was out of pocket, in an inebriated state , during the nuclear alert) meant to warn Soviet leaders that they had better back down before events spun out of control.

Preventive war is altogether different. Starting one against Iraq in 2003 was perhaps the biggest foreign policy blunder of the past century, a point Trump himself has repeated often, and initiating one against North Korea would be the height of folly, especially because it could easily entail nuclear strikes or quickly lead to a nuclear exchange.

Nobody in the U.S. military’s nuclear ranks signed up for this. Our implied “contract” was to deter an enemy attack, and if deterrence failed, to destroy the enemy’s warfighting ability. Nuking another country just because it seeks to acquire nuclear weapons enjoys virtually zero support from U.S. nuclear troops. I know the culture well, and I stay in touch with many dozens of former missileers. Preventive war represents the antithesis of everything they stand for. (Preventive strikes against North Korea would be carried out by submarine and bomber crews, because land-based Minuteman missiles would have to fly over Russia and China, possibly triggering mistaken retaliation by these countries.)

It would also be disastrous. Intelligence on an adversary’s capabilities and intentions may be dead wrong, as it was before the Iraq invasion. Threatening preventive war would accelerate the other side’s effort to get the bomb to bolster its security and deter Trump, which means it encourages proliferation among insecure nations. A willingness to launch first in a simmering confrontation also works to destabilize crises. And such a provocation would violate the United Nations charter, which allows for the threat or use of force only in self-defense when there is no other recourse. The first-strike execution of any of the current U.S. nuclear war plans, even smaller options such as the plan for North Korea (with about 80 nuclear aim points), would also violate the laws of war and international humanitarian law. As a matter of policy, the Defense Department subscribes to these legal strictures. Nuclear commanders could be hauled into court at The Hague on charges of war crimes.

Yet Trump indulges in issuing such threats, and he has unchecked authority to order a preventive nuclear strike against any nation he wants with a single verbal direction to the Pentagon war room. Under the current nuclear strike protocol, he can consult any and all — or none — of his national security advisers, and no one can legally countermand his order. If he gave the green light using his nuclear codes, a launch order the length of a tweet would be transmitted and carried out within a few minutes . I could fire my missiles 60 seconds after receiving an order. There would be no recalling missiles fired from silos and submarines.

I believe the nuclear commanders at all levels would obey such an order, despite deep misgivings about its wisdom and legality. The military’s thorough subordination to civilian control and deeply ingrained attitude of deference to presidential direction; its well-greased and practiced protocols from top to bottom of the nuclear chain of command, geared to carry out his orders quickly (and to pressure a hesitant president to give the order) — as well as widespread ignorance among the rank and file about the dubious legality of striking first — leave little doubt in my mind that a presidential decision to strike a preventive blow, however misguided and reckless, would not be thwarted. It might be opposed strenuously by his advisers if they had a chance to weigh in, but in the end, they would acquiesce.

Which means there is a silver lining in having a president who is unfit to wield absolute power over the fate of the world: It has engendered serious and urgent proposals to strengthen checks and balances on nuclear decision-making. One prominent idea circulating in Congress, spearheaded by Democrats Ted Lieu and Edward Markey, is to require a congressional declaration of war with specific authorization for the deployment of nuclear weapons before a president could order their first use. Other proposals would require a consensus among additional top leaders for either first or second use.

To reinforce these steps, the United States should officially adopt a policy of no first use (and educate missileers about the illegality of preventive first use), eliminate “use or lose” weapons such as the vulnerable silo-based missile force, and make big improvements in nuclear command-and-control to increase warning and decision time. That, not the weapons, should be the centerpiece of our trillion-dollar program of nuclear modernization.

Ultimately, the only true solution is for the nine countries that possess nuclear weapons to eliminate them all — global zero. None of them, except possibly Israel, have strong checks and balances on their leaders’ ability to use nuclear weapons. If we do not eliminate these arsenals in our lifetime, then they almost certainly will be used in our lifetime — on purpose or by accident.